In general, a lithographic printing plate is composed of an oleophilic image area accepting ink and a hydrophilic non-image area accepting dampening water (fountain solution) in the process of printing. Lithographic printing is a printing method utilizing the nature of water and oily ink to repel with each other and comprising rendering the oleophilic image area of the lithographic printing plate to an ink-receptive area and the hydrophilic non-image area thereof to a dampening water-receptive area (ink-unreceptive area), thereby making a difference in adherence of the ink on the surface of the lithographic printing plate, depositing the ink only to the image area, and then transferring the ink to a printing material, for example, paper.
In order to produce the lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer (image-recording layer) has heretofore been broadly used. Ordinarily, the lithographic printing plate is obtained by conducting plate making according to a method of exposing the lithographic printing plate precursor through an original, for example, a lith film, and then while leaving the part forming the image area of the image-recording layer, removing the other unnecessary image-recording layer by dissolving with an alkaline developer or an organic solvent to reveal the hydrophilic surface of support.
In the hitherto known plate making process of lithographic printing plate precursor, after exposure, the step of removing the unnecessary image-recording layer by dissolving, for example, with a developer is required. However, it is one of the subjects to save or simplify such an additional wet treatment described above. Particularly, since disposal of liquid wastes discharged resulting from the wet treatment has become a great concern throughout the field of industry in view of the consideration for global environment in recent years, the demand for the solution of the above-described subject has been increased more and more.
As one of simple plate making methods in response to the above-described requirement, a method referred to as on-machine development has been proposed wherein a lithographic printing plate precursor having an image-recording layer capable of being removed in the unnecessary areas during a conventional printing process is used and after imagewise exposure, the image-recording layer corresponding to the non-image area is removed on a printing machine to prepare a lithographic printing plate.
Specific methods of the on-machine development include, for example, a method of using a lithographic printing plate precursor having an image-recording layer that can be dissolved or dispersed in dampening water, an ink solvent or an emulsion of dampening water and ink, a method of mechanically removing an image-recording layer by contact with a roller or a blanket cylinder of a printing machine, and a method of lowering cohesion of an image-recording layer or adhesion between an image-recording layer and a support upon penetration of dampening water, ink solvent or the like and then mechanically removing the image-recording layer by contact with a roller or a blanket cylinder of a printing machine.
In the invention, unless otherwise indicated particularly, the term “development processing step” means a step of using an apparatus (ordinarily, an automatic developing machine) other than a printing machine and removing an unexposed area in an image-recording layer of a lithographic printing plate precursor upon contact with liquid (ordinarily, an alkaline developer) thereby revealing a hydrophilic surface of support. The term “on-machine development” means a method or a step of removing an unexposed area in an image-recording layer of a lithographic printing plate precursor upon contact with liquid (ordinarily, printing ink and/or dampening water) by using a printing machine thereby revealing a hydrophilic surface of support.
On the other hand, digitalized technique of electronically processing, accumulating and outputting image information using a computer has been popularized in recent years, and various new image-outputting systems responding to the digitalized technique have been put into practical use. Correspondingly, attention has been drawn to a computer-to-plate (CTP) technique of carrying digitalized image information on highly converging radiation, for example, a laser beam and conducting scanning exposure of a lithographic printing plate precursor with the radiation thereby directly preparing a lithographic printing plate without using a lith film. Thus, it is one of the important technical subjects to obtain a lithographic printing plate precursor adaptable to the technique described above.
As described above, in recent years, the simplification of plate making operation and the realization of dry system and non-processing system have been further strongly required from both aspects of the consideration for global environment and the adaptation for digitization.
As lithographic printing plate precursors adapting to the above-described requirements, lithographic printing plate precursors having a photopolymerizable and/or heat-polymerizable image-recording layer are used. In such a lithographic printing plate precursor, a protective layer (overcoat layer) is provided on the image-recording layer for the purpose of imparting an oxygen blocking property, preventing occurrence of scratch on the image-recording layer, preventing ablation caused at the time of exposure with a high illuminance laser beam or the like. As a material for the protective layer, polyvinyl alcohol, polyvinyl pyrrolidone, polyvinyl imidazole, a water-soluble acrylic resin (for example, polyacrylic acid), gelatin, gum arabic or a cellulose polymer (for example, carboxymethyl cellulose) is exemplified. It is also known that the oxygen blocking property is improved and the occurrence of scratch on the image-recording layer is more prevented by means of adding a stratiform compound, for example, mica (see, for example, JP-A-2001-171250 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-2005-119273 (corresponding to US2005/0069811 A1)).
However, when the stratiform compound is used in the protective layer, one or more of the performances, for example, ink-receptive property at the beginning of printing, ink-receptive property during printing, developing property or on-machine development property may decrease in sometimes. In particular, when the stratiform compound is added to a water-soluble polymer, for example, polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylic acid or a cellulose polymer, which is suitably used in the protective layer, because of the good oxygen blocking property, the polymerization is apt to proceed resulting in the improvement in printing durability, but the development property and ink-receptive property may decrease in many cases. Therefore, the solution of such a problem is one of the important technical tasks.
In order to improve the decrease in ink-receptive property, it is proposed that a phosphonium compound is added to an image-recording layer or a protective layer (see, for example, JP-A-2006-297907 (corresponding to US2006/0194150 A1) and JP-A-2007-50660 (corresponding to US2007/0042293 A1)). However, these proposals are still insufficient in view of achieving excellent levels in all of the printing durability, on-machine development property and ink-receptive property.